Nuts and Volts - November 2013

an LED to indicate basic functionality, and a screw terminal array configured to behave as digitally compatible switching inputs via some pull-up resistors. I don’t mean to understate things, yet the circuit design is just that simple. This is certainly a case where the firmware handles the brunt of the task.

All project files are available at the article link, so the schematic diagram presented here is to be used as an outline of behavior. Notice how the schematic generally resembles the image of the original prototype.

There are plenty of standards and opinions regarding circuit design. Keeping things simple and straightforward is usually the best policy to promote understanding and even dreaded troubleshooting. The schematic’s only difference is it has more input switches in the form of pull-up resistor-connected pin headers (top of schematic: S1-S16) and the addition of an optional voltage regulator (IC3).

The switches work as simple closure contacts. For example, R2 (1K ohm to 39K ohms, depending on signal skew and transient rejection desired) is connected serially to the voltage source and to switch S1 (pin header). The other connection is S1 to ground.

When S1 is closed (terminal is shorted to ground), the voltage seen at the junction of R2 and S1 changes from the nominal high voltage (five volts, in this case) to ground (0 Vdc). The PIC will interpret this voltage change as an input logic transition from 1 to 0. Thus, we have a negative logic switch. As long as our firmware knows this, everything will work fine.

Resistor R18 (1K ohms-5K ohms, depending on desired brightness) and diode D4 are the components used to connect an “idiot light” LED to our PIC.

Again, this will be a logic 0 driven output. Other than the output via a USB connection, this will be our only indication of functionality.

The example firmware is written so that the LED will illuminate on device startup initialization and on key presses (switch closures). In my humble opinion, there

November 2013 29 ■ FIGURE 2. Schematic.

ITEM DESCRIPTION MFG PART# SOURCE / NOTES

C2 CAP/TANT 4. 7 µF 16V 20% 1206 F931C475MAA Nichicon C4 CAP/CER 0.47 µF 10V 20% 0603 C1608X5R1A474M TDK Corporation D1 DIODE SWITCHING 75V SOD323 1N4448WS-7-F Diodes, Inc.

D2-3 DIODE ZENER 3.9V SOT23-3 BZX84C3V9-7-F Diodes, Inc.

D4 LED CHIPLED 633NM RED 0603 LS Q976-NR-1 OSRAM Opto Semiconductors Inc / Not required for operation IC1 IC MCU 8-BIT 32 KB FLASH 28SOIC PIC18F2550-I/SO Microchip Technology IC3 REG LDO 5V 0.1A TO92-3 L78L05ACZ ST Microelectronics / Not required for operation JUSB CONN USB A SMD 1001-011-01101 CNC Tech JPRG CONN HEADER .100 5-POS PEC05SAAN Sullins Connector Solutions / Programming Header POW 9V BATTERY CONNECTOR 84-4 Keystone Electronics / Not required for operation R1-17 RES 39.0K OHM 1/10W 1% 0603 RC0603FR-0739KL Yageo / Valid range from 1K ohm to 100K ohm R18 RES 3.30K OHM 1/10W 1% 0603 RC0603FR-073K3L Yageo / Valid range from 100 ohm to 5K ohm S1-16 MOMENTARY SWITCH SPST-NO GPB024A05BB CW Industries / Not required for operation Y1 20 MHz CERAMIC RESONATOR PBRC20.00MR50X000 AVX Corp Note: Generally, most of these part values and manufacturer part numbers can be substituted.